twenty_four.cpp

#include <algorithm>
#include <stdio.h>

typedef float (*op_t)(float, float);

float op_add (float l, float r) { return l + r; }
float op_sub (float l, float r) { return l - r; }
float op_subr(float l, float r) { return r - l; }
float op_mul (float l, float r) { return l * r; }
float op_div (float l, float r) { return l / r; }
float op_dvr (float l, float r) { return r / l; }

op_t  ops  [] = {op_add, op_sub, op_subr, op_mul, op_div, op_dvr};
char* ops_s[] = {   "+",    "-",     "--",  "*",    "/",  "\\"};
int op_count = 6;

int main() {
    
    float nums[] = {1, 3, 4, 6};
    
    std::sort(nums, nums + 4);
    int iteration_count = 0;
    do {
        for (int o0=0; o0<op_count; o0++) {
            for (int o1=0; o1<op_count; o1++) {
                for (int o2=0; o2<op_count; o2++) {
                    
                    iteration_count++;
                    float r = nums[0];
                    r = ops[o0](r, nums[1]);
                    r = ops[o1](r, nums[2]);
                    r = ops[o2](r, nums[3]);
                    
                    if (r == 24 ) {
                        printf("(((%d %s %d) %s %d) %s %d) = %f\n", 
                               (int)nums[0], ops_s[o0], 
                               (int)nums[1], ops_s[o1], 
                               (int)nums[2], ops_s[o2],
                               (int)nums[3], r);
                        fflush(stdout);
                    }
                    
                }
            }
        }
    } while (std::next_permutation(nums, nums+4));
    
    printf("iteration_count: %d\n", iteration_count);
    return 0;
}